(a) Field of the Invention
The present invention relates to a silver sol excellent in storage stability, a method for preparing the same, a coating material for forming a transparent conductive film and a transparent conductive film formed from the coating material and more particularly to a transparent conductive film which has high transparency and is excellent in electrical conductivity and, in particular, electromagnetic shielding properties as well as a silver sol used for preparing the transparent conductive film, a method for preparing the same and a coating material for forming the film.
(b) Description of the Prior Art
Cathode-ray tubes including those used in televisions and used as a means for computer display have conventionally been required to have high resolution and good antistatic properties. However, they have recently been required to also show excellent electromagnetic shielding properties in addition to the foregoing requirements. For instance, the magnitude of the voltage applied to the cathode-ray tube is increasingly high as the size of the tube increases. This in turn leads to the generation of a high voltage-static electricity and accordingly, the picture plane becomes whitish due to dusts adhered to the surface of the tube through electrostatic attraction. In case of computer display, an operator manipulates a computer at a position in the proximity to the surface of the cathode-ray tube and accordingly, it has been feared that the health of the operator may be adversely affected by the static electricity and electromagnetic waves generated by the cathode-ray tube.
Rules concerning the electromagnetic shielding properties of the cathode-ray tube have gradually been set in various countries. Accordingly, there has conventionally been used, as a means for achieving electromagnetic shielding, a method for forming a transparent conductive film on the cathode-ray tube surface. An example of such transparent conductive film-forming methods comprises applying a transparent conductive film such as indium tin oxide (ITO) or antimony-doped tin oxide (ATO) onto the cathode-ray tube surface according to, for instance, a sputtering or vapor deposition technique.
However, the ITO films formed by such vapor deposition and sputtering methods necessarily lead to an increase in cost since the formation thereof requires the use of quite expensive film-forming installations and these methods do not permit mass production. For this reason, there has been a strong demand for the development of a novel transparent conductive film as well as a process for forming the same, which permits the reduction of the production cost and mass production of the film.
On the other hand, there have been established various specifications for the electromagnetic shielding properties of cathode-ray tubes, for instance, TCO (Tianstemanners Central Organisation) Guide Lines established by Swedish Central Labor's Society. According to these guide lines, the cathode-ray tube should have a leaked field strength of not more than 1.0 V/m. To satisfy this requirement, the surface resistance of the conductive film should be reduced to a level of not more than 1.times.10.sup.3 .OMEGA./.quadrature..
Under such circumstances, there have widely been used sol-gel methods since they permit the reduction of its production cost and the formation of conductive films having a low surface resistance.
The sol-gel technique is quite effective since the technique permits uniform coating of the whole surface of a large-sized substrate at a relatively low cost and it can impart, to the resulting coating film, functions of controlling optical and electromagnetic characteristic properties thereof. In this method, a solution of a metal alkoxide in an alcohol is subjected to hydrolysis and poly-condensation at a low temperature in the presence of water and an acidic catalyst to give a sol. The metal alkoxide: M(OR).sub.n carries an alkoxyl group (--OR) having strong electronegativity and an M--O bond having a high polarity. Therefore, M is susceptible to nucleophilic attack, is quite reactive and easily undergoes a reaction (hydrolysis) with H.sub.2 O. After the hydrolysis, the compound subsequently undergoes a poly-condensation reaction which is accompanied by a dehydration reaction and de-alcohol reaction.
Hydrolysis: EQU M(OR).sub.n +xH.sub.2 O.fwdarw.M(OH).sub.x (OR).sub.n-x +xROH
Polycondensation:
Dehydration Reaction: EQU --M--OH+HO--M--.fwdarw.--M--O--M--+H.sub.2 O
De-alcohol Reaction: EQU --M--OH+RO--M--.fwdarw.--M--O--M--+ROH
The resulting sol further undergoes polymerization when applied onto the surface of a substrate by, for instance, a dip coating, spin coating or spray coating method and then heated to thus form a gelled metal oxide film. In general, a solution of tetraethoxysilane (TEOS: Si(C.sub.2 H.sub.5 O).sub.4) as a metal alkoxide in ethanol as a solvent is subjected to hydrolysis and then poly-condensation in the presence of water and HCl or HNO.sub.3 as an acidic catalyst to give such a sol. The coating properties of the sol may be controlled by adjusting, for instance, the sol concentration and the rate of evaporation through addition of, for instance, a high boiling point solvent to the coating solution, i.e., the sol. After the application of the sol, the coated film is heated to a temperature of not less than 180.degree. C. to form a silicon oxide film.
The conductivity of the silicon oxide film carrying silanol groups formed by the sol-gel technique is originated from ionic conduction through hydronium ions formed from water molecules adsorbed on the silanol groups and therefore, the surface resistivity thereof is quite susceptible to moisture. To eliminate such a problem, there has been proposed a method in which a hygroscopic salt is incorporated in the film, but such a film would be insufficient in the stability, with time, (or durability) of the antistatic effect because of the conversion of silanol groups into siloxane groups.
To solve the foregoing problem, there has been proposed a technique in which conductive fine particles of antimony-doped tin oxide (ATO: Antimony Tin Oxide) having a diameter of about 10 nm are dispersed in a sol of a metal alkoxide to thus form a tin oxide film contaminated with ATO. This film can ensure a stable or long-lasting antistatic effect due to the electron conduction of ATO. Alternatively, conductive fine particles of ITO are sometimes used in place of ATO in order to obtain a film having a lower resistance.
However, even a conductive film formed by this method still has a high surface resistance on the order of more than 10.sup.3 .OMEGA./.quadrature. and is still insufficient in the electromagnetic shielding effect. The reason why this method cannot provide a film having a reduced surface resistance would be as follows. ATO consists of fine particles of a metal oxide, it is applied onto the surface of a cathode-ray tube and then heated to form a film for electromagnetic shielding. However, the cathode-ray tube is sensitive to temperature and accordingly, the coated film cannot be heated at an unreasonably high temperature. For this reason, such a film is in general heated at a temperature on the order of about 150.degree. C. The metal oxide fine particles do not cause any sintering therebetween at that temperature. Therefore, the conductivity of the resulting film simply relies on the contact between particles and this would not permit any reduction in the surface resistance.
Contrary to this, fine particles are sintered in the film formed by the sputtering technique and thus the sputtering technique permits the reduction of the surface resistivity to a level of not more than 10.sup.3 .OMEGA./.quadrature., but the resulting film is quite expensive as has been described above and has not yet been put to practical use.